Imporvement: functions in SafeMath contract overloaded to accept cust…

…om error messages.

contracts/math/SafeMath.sol

@@ -24,8 +24,21 @@ library SafeMath {
      * - Addition cannot overflow.
      */
     function add(uint256 a, uint256 b) internal pure returns (uint256) {
+        return add(a, b, "SafeMath: addition overflow");
+    }
+
+    /**
+     * @dev Returns the addition of two unsigned integers, reverting with custom message on
+     * overflow.
+     *
+     * Counterpart to Solidity's `+` operator.
+     *
+     * Requirements:
+     * - Addition cannot overflow.
+     */
+    function add(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
         uint256 c = a + b;
-        require(c >= a, "SafeMath: addition overflow");
+        require(c >= a, errorMessage);
 
         return c;
     }
@@ -40,7 +53,20 @@ library SafeMath {
      * - Subtraction cannot overflow.
      */
     function sub(uint256 a, uint256 b) internal pure returns (uint256) {
-        require(b <= a, "SafeMath: subtraction overflow");
+        return sub(a, b, "SafeMath: subtraction overflow");
+    }
+
+    /**
+     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
+     * overflow (when the result is negative).
+     *
+     * Counterpart to Solidity's `-` operator.
+     *
+     * Requirements:
+     * - Subtraction cannot overflow.
+     */
+    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
+        require(b <= a, errorMessage);
         uint256 c = a - b;
 
         return c;
@@ -56,6 +82,19 @@ library SafeMath {
      * - Multiplication cannot overflow.
      */
     function mul(uint256 a, uint256 b) internal pure returns (uint256) {
+        return mul(a, b, "SafeMath: multiplication overflow");
+    }
+
+    /**
+     * @dev Returns the multiplication of two unsigned integers, reverting with custom message on
+     * overflow.
+     *
+     * Counterpart to Solidity's `*` operator.
+     *
+     * Requirements:
+     * - Multiplication cannot overflow.
+     */
+    function mul(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
         // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
         // benefit is lost if 'b' is also tested.
         // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
@@ -64,7 +103,7 @@ library SafeMath {
         }
 
         uint256 c = a * b;
-        require(c / a == b, "SafeMath: multiplication overflow");
+        require(c / a == b, errorMessage);
 
         return c;
     }
@@ -81,8 +120,23 @@ library SafeMath {
      * - The divisor cannot be zero.
      */
     function div(uint256 a, uint256 b) internal pure returns (uint256) {
+        return div(a, b, "SafeMath: division by zero");
+    }
+
+    /**
+     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
+     * division by zero. The result is rounded towards zero.
+     *
+     * Counterpart to Solidity's `/` operator. Note: this function uses a
+     * `revert` opcode (which leaves remaining gas untouched) while Solidity
+     * uses an invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     * - The divisor cannot be zero.
+     */
+    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
         // Solidity only automatically asserts when dividing by 0
-        require(b > 0, "SafeMath: division by zero");
+        require(b > 0, errorMessage);
         uint256 c = a / b;
         // assert(a == b * c + a % b); // There is no case in which this doesn't hold
 
@@ -101,7 +155,22 @@ library SafeMath {
      * - The divisor cannot be zero.
      */
     function mod(uint256 a, uint256 b) internal pure returns (uint256) {
-        require(b != 0, "SafeMath: modulo by zero");
+        return mod(a, b, "SafeMath: modulo by zero");
+    }
+
+    /**
+     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
+     * Reverts with custom message when dividing by zero.
+     *
+     * Counterpart to Solidity's `%` operator. This function uses a `revert`
+     * opcode (which leaves remaining gas untouched) while Solidity uses an
+     * invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     * - The divisor cannot be zero.
+     */
+    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
+        require(b != 0, errorMessage);
         return a % b;
     }
 }